In a typical switching mode power supply, the switching frequency is either fixed or increases as the load becomes lighter, often resulting in poor efficiency at light loads and poor average efficiency. Switching mode power supplies also typically include current controllers that limit the peak current to a fixed value, often causing acoustic noise at light loads.
In accordance with the teachings described herein, a switching mode power supply with a multi-mode controller is provided. The switching mode power supply may include a transformer having a primary winding and a secondary winding to supply power to a load. A feedback circuit may be included to generate a feedback signal that varies in relation to the load on the secondary winding. The multi-mode controller may include a switching circuit, a frequency control circuit and a current limiting circuit. The switching circuit may be coupled to the primary winding to control current flow through the primary winding. The frequency control circuit may control a switching frequency of the switching circuit based on the feedback signal. The current limiting circuit may limit current flow through the primary winding by causing the switching circuit to suspend current flow through the primary winding when the current reaches a peak current limit that is set based on the feedback signal.
A method for controlling a switching mode power supply may include the following steps: regulating an output voltage of the switching mode power supply by switching a transformer on and off at a switching frequency; generating a feedback signal that varies in relation to a load coupled to the output voltage; controlling the switching frequency based on the feedback signal; and limiting a peak current through a primary winding of the transformer based on the feedback signal.